Exhaust treatment system implementing selective doc bypass

ABSTRACT

An exhaust treatment system for use with a power system is disclosed. The exhaust treatment system may have an SCR device ( 32 ), and an oxidation device ( 26 ) located upstream of the SCR device ( 32 ) to convert NO to NO2. The exhaust treatment system may also have an exhaust passageway ( 14 ) extending from an exhaust source ( 10 ) to the oxidation device ( 26 ), and a bypass passageway ( 24 ) extending from the exhaust passageway at a location upstream of the oxidation device to the exhaust passageway at a location downstream of the oxidation device ( 26 ). The exhaust treatment system may further have a valve element ( 20 ) configured to selectively direct exhaust from the exhaust source ( 10 ) through the oxidation device ( 26 ) and through the bypass passageway ( 24 ), at least one sensor configured to sense operating parameters of the exhaust source ( 10 ), and a controller ( 36 ) in communication with the valve element ( 20 ). The controller ( 36 ) may be configured to move the valve element ( 20 ) in response to an estimated ratio of NO to NO2 based on sensed operating parameters of the exhaust source ( 10 ).

TECHNICAL FIELD

The present disclosure is directed to an exhaust treatment system and,more particularly, to an exhaust treatment system that implementsselective Diesel Oxidation Catalyst (DOC) bypass.

BACKGROUND

Internal combustion engines, including diesel engines, gasoline engines,gaseous fuel-powered engines, and other engines known in the art exhausta complex mixture of air pollutants. These air pollutants may becomposed of gaseous compounds such as, for example, the oxides ofnitrogen (NOx). Due to increased awareness of the environment, exhaustemission standards have become more stringent, and the amount of NOxemitted from an engine may be regulated depending on the type of engine,size of engine, and/or class of engine. In order to ensure compliancewith the regulation of these compounds, some engine manufacturers haveimplemented a strategy called Selective Catalytic Reduction (SCR).

SCR is a process where gaseous or liquid reductant (most commonly urea)is added to the exhaust gas stream of an engine and is absorbed onto acatalyst. The reductant reacts with NOx in the exhaust gas to form H₂Oand N₂. Although SCR can be effective, it is most effective when aconcentration of NO to NO₂ supplied to the SCR is about 1:1. In order toachieve this optimum ratio, a Diesel Oxidation Catalyst (DOC) is oftenlocated upstream of the SCR to convert NO to NO₂.

In addition to facilitating the reduction process of the SCR, theproduced NO₂ also facilitates the combustion of particulate matter.Specifically, a particulate trap is commonly used to collect unburnedparticulates also known as soot. Over time, the particulate matterbuilds up in the trap and, if left unchecked, the particulate trap couldnegatively affect performance of the engine. As such, the particulatematter collected by the trap must be periodically removed through aprocess called regeneration. To regenerate the particulate trap, aliquid catalyst (typically diesel fuel) is injected into the exhaustflow upstream of the trap. The fuel, in the presence of NO₂, ignites andburns away the particulate matter.

During operation of an associated engine, it may be desirable toselectively divert exhaust away from the DOC (i.e., bypass the DOC). Forexample, during some engine operating conditions, the ratio of NO to NO₂may naturally be about 1:1. In this situation, if all of the exhaust ispassed through the DOC, the ratio of NO to NO₂ could actually exceed thedesired 1:1 ratio and reduce the effectiveness of the SCR process. Thus,under some conditions, the exhaust flow can be directed to bypass theDOC. In another example, the DOC is only necessary during trapregeneration events. In this example, in order to conserve the DOC, theexhaust flow can again be directed to bypass the DOC duringnon-regeneration events.

One system implementing DOC bypass is described in Japanese Laid-OpenPatent Application JP 2005-2968 A (the '968 publication) by MitsubishiFuso, published Jan. 6, 2005. The '968 publication discloses an exhaustgas purifying system having an oxidation catalyst, a temperature sensor,a bypass path, a bypass switching device, and an SCR device. The systemis designed to create a 1:1 ratio of NO:NO₂ in an exhaust flow. Thesystem estimates a ratio of NO:NO₂ based on a sensed exhaust gastemperature. The oxidation catalyst converts NO to NO₂, and the SCRdevice converts NO and NO₂ to N₂ in the presence of ammonia. The SCRdevice operates most efficiently when the ratio of NO:NO₂ is 1:1. At acertain temperature, above which the ratio of NO:NO₂ in the exhaust isestimated to be 1:1, the bypass switching device diverts exhaust flow sothat the exhaust gas flows through the bypass path, and not through theoxidation catalyst. In this way, the system aims to prevent excessiveNO₂ in the exhaust gas flow. The '968 publication also discloses thepossibility of employing a NOx sensor to directly sense a NO:NO₂ ratio.

Although the exhaust gas purifying apparatus of the '968 publication maydisclose a method of operation that aims to achieve a 1:1 ratio of NO toNO₂ in an exhaust flow, it may be limited. For example, there currentlyare no commercially available NOx sensors that satisfy desiredperformance requirements to measure NOx effectively and quickly enoughto provide real-time control over NO:NO₂ ratio in an exhaust flow.Additionally, estimating an NO:NO₂ ratio based on a measurement ofexhaust gas temperatures and other exhaust parameters may not accuratelyreflect the NO:NO₂ ratio, because, for example, changes in engineoperating parameters may cause changes in an exhaust gas NO:NO₂ ratio,but not an exhaust gas temperature. Similarly, changes in engineoperating parameters may cause changes in exhaust gas temperature butnot in an exhaust gas NO:NO₂ ratio.

The system of the present disclosure solves one or more of the problemsset forth above.

SUMMARY OF THE DISCLOSURE

One aspect of the present disclosure is directed to an exhaust treatmentsystem. The exhaust treatment system may include an SCR device, and anoxidation device located upstream of the SCR device to convert NO toNO₂. The exhaust treatment system may also include an exhaust passagewayextending from an exhaust source to the oxidation device, and a bypasspassageway extending from the exhaust passageway at a location upstreamof the oxidation device to the exhaust passageway at a locationdownstream of the oxidation device. The exhaust treatment system mayfurther include a valve element configured to selectively direct exhaustfrom the exhaust source through the oxidation device and through thebypass passageway, at least one sensor configured to sense operatingparameters of the exhaust source, and a controller in communication withthe valve element. The controller may be configured to move the valveelement in response to an estimated ratio of NO to NO₂ based on sensedoperating parameters of the exhaust source.

Another aspect of the present disclosure is directed to a method oftreating exhaust. The method may include generating a flow of exhaust,treating at least a portion of the flow of exhaust by a catalyst, anddirecting the flow of exhaust through an SCR device. The method may alsoinclude estimating a ratio of NO to NO₂ in the flow of exhaust based onsensed operating parameters of a power source that generates the flow ofexhaust, and changing an amount of the at least a portion in response tothe estimation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic and diagrammatic illustration of an exemplarydisclosed power system; and

FIG. 2 is a flowchart depicting an exemplary disclosed operation of thepower system of FIG. 1.

DETAILED DESCRIPTION

FIG. 1 illustrates a power source 10 having an exemplary embodiment ofan exhaust treatment system 12. Power source 10 may include an enginesuch as, for example, a diesel engine, a gasoline engine, a gaseousfuel-powered engine, or any other engine apparent to one skilled in theart. Power source 10 may also include any non-engine source of power,such as a furnace. Power source 10 may combust a mixture of air and fuelto produce a power output and an exhaust gas flow. The exhaust gas flowfrom power source 10 may be diverted through exhaust treatment system12.

Exhaust treatment system 12 may include components that cooperate, byway of a main passageway 14, to treat the exhaust gas flow from powersource 10. In particular, exhaust treatment system 12 may include adiesel oxidation catalyst (DOC) 26, an SCR device 32, and a ureainjection unit 30. Exhaust treatment system 12 may also include a bypasscircuit having a bypass valve 20 and bypass passageway 24.

DOC 26 may be located within main passageway 14 and include a porousceramic honeycomb-like or metal mesh substrate. The substrate may becoated with a material such as, for example, a precious metal, thatcatalyzes a chemical reaction to alter the chemical composition ofexhaust gas. For example, DOC 26 may include platinum or vanadium tofacilitate the conversion of NO constituents into NO₂, which may be moresusceptible to catalytic treatment in SCR device 32.

SCR device 32 may be disposed in main passageway 14 downstream of DOC26. SCR device 32 may chemically reduce NOx into N₂ in the presence of acatalyst such as ammonia or urea. Efficiency of NOx reduction by SCRdevice 32 may be at least partially dependent on the ratio of NO to NO₂in the exhaust. In particular, NOx reduction by SCR device 32 may bemost efficient when the ratio of NO to NO₂ in the exhaust is about 1:1.In a lean gas flow, a lean NOx SCR device 32 may need reductants for thechemical reaction and may utilize a reductant injector to introduce thereductant into the lean gas flow. Reductants employed may be dieselfuel, ethanol, blended fuels, or any other reductant known in the art.SCR device 32 may include a catalyst support material and a metalpromoter dispersed within the catalyst support material. The catalystsupport material may include at least one of alumina, zeolite,aluminophosphates, hexaluminates, aluminosilicates, zirconates,titanosilicates, and titanates. The catalyst support material may alsoinclude at least one of alumina and zeolite, and the metal promoter mayinclude silver metal (Ag). Combinations of these materials may be used,and the catalyst material may be chosen based on the type of fuel used,the ethanol additive used, the air to fuel-vapor ratio desired, and/orfor conformity with environmental standards. One of ordinary skill inthe art will recognize that numerous other catalyst compositions may beused without departing from the scope of this disclosure. More than oneSCR device 32 may be included in main passageway 14.

Urea injection unit 30 may be located adjacent to or upstream of SCRdevice 32 to inject urea directly into SCR device 32 and/or into mainpassageway 14. The injected urea may be broken down into ammonia, whichmay be retained within SCR device 32. The ammonia stored in SCR device32 may be used to reduce the amount of NO_(x) in the exhaust gasespassing through SCR device 32 by converting NO₂ to N₂. Alternatively oradditionally, other agents suitable for reducing NO_(x) may be injectedinto main passageway 14 and/or SCR device 32.

Bypass valve 20 may be fluidly connected to main passageway 14 andbypass passageway 24 at a point upstream of DOC 26. Bypass valve 20 maybe any commonly known three-way valve capable of directing flow invariable proportion between two separate passageways (i.e. between mainpassageway 14 and bypass passageway 24). Alternatively, bypass valve 20may be a two-way valve (not shown) located within bypass passageway 24.Bypass valve 20 may include a valve element 22 configured to control theamount of exhaust gas delivered to DOC 26. In particular, valve element22 may be movable between a first, “open” position, at whichsubstantially all of the exhaust gas flow from power source 10 isdirected to flow through bypass passageway 24, toward a second, “closed”position, at which all of the exhaust gas flow from power source 10 isdirected to flow through DOC 26. Valve element 22 may also be positionedat any intermediate position between the open and closed positions, todirect portions of the exhaust gas flow to both DOC 26 and bypasspassageway 24. Valve element 22 may include a spool valve element, aball valve element, a globe valve element, a butterfly valve element, orany other suitable type of valve element known in the art. Bypass valve20 may include means for automatically moving valve element 22 inresponse to a control signal. Bypass passageway 24 may extend from mainpassageway 14 a point upstream of DOC 26 to main passageway 14 at apoint downstream of DOC 26, and may provide an alternate path forexhaust flow from power source 10.

A control system 34 may regulate the operation of bypass valve 20 inresponse to one or more inputs. In particular, control system 34 mayinclude a controller 36 that communicates with bypass valve 20 by way ofa communication line 40, and with sensor 38 by way of a communicationline 42. In response to an input from sensor 38, and/or from othersources such as power source 10 and/or DOC 26, controller 36 may adjusta setting of valve element 22.

Controller 36 is shown in FIG. 1 as a single controller, and it mayinclude one or more microprocessors that include a means for controllingan operation of exhaust treatment system 12. Alternatively, controller36 may be one or more controllers, each assigned to control a subsystem,and in communication with each other, for example a controllerconfigured to control power source 10, and a separate controllerconfigured to control exhaust treatment system 12. Numerous commerciallyavailable microprocessors may be configured to perform the functions ofcontroller 36. It should be appreciated that controller 36 mayalternatively embody a general engine control unit (ECU) capable ofcontrolling numerous functions, including power source 10 and exhausttreatment system 12. Controller 36 may include all of the componentsrequired to run an application such as, for example, a memory, asecondary storage device, and a processor, such as a central processingunit or any other means known in the art for controlling bypass valve 20and sensor 38. Various other known circuits may be associated withcontroller 36, including power supply circuitry, signal-conditioningcircuitry, solenoid driver circuitry, communication circuitry, and otherappropriate circuitry.

Controller 36 may receive and store in memory communication from varioussensors and components commonly known in the art, such as, for example,sensor 38, including measurements of, for example, exhaust gas NOxcomposition and concentrations, power source fuel/air settings, powersource operating speed, power source load, power source fuel injectionprofile, other power source operating parameters, and/or DOC 26operating temperature. Controller 36 may analyze and compare receivedand stored data, and, based on instructions and data stored in memory orinput by a user, determine whether action is required. For example,controller 36 may compare received values with target values stored inmemory, and, based on the results of the comparison, controller 36 maytransmit signals to adjust bypass valve 20.

Controller 36 may include memory means known in the art for storing datarelating to engine operation. The data may be stored in the form of oneor more maps that describe relationships between various power source 10and/or DOC 26 operating parameters and resulting power source 10 exhaustgas compositions. Each of these maps may be in the form of tables,graphs, and/or equations, and include a compilation of data collectedfrom lab and/or field operation of power source 10 and DOC 26. Thesemaps may be generated by performing instrumented tests on the operationof power source 10 and DOC 26 under a variety of operating conditions,while varying parameters such as power source fuel/air settings, powersource operating speed, power source load, power source fuel injectionprofile, other power source operating parameters, and while measuringDOC 26 operating temperature and exhaust gas NO:NO₂ ratio. Data from thetests may be logged, and may show correlation, for example, among one ormore power source and/or DOC operating parameters and exhaust gas NOxcomposition, including a NO:NO₂ ratio. Additionally, controller 36 maybe capable of updating the maps based on measured operating conditionsof power source 10 and DOC 26, which may allow controller 36 to adjustthe maps to match the particular operating characteristics and modes ofan individual power source 10 and DOC 26. Controller 36 may referencethese maps and control the position of bypass valve 20 to bring theoperation of exhaust treatment system 12 in line with desired values.

Controller 36 may also contain one or more virtual models of exhausttreatment system 12. A virtual model may contain information such astables, graphs, and/or equations, and include a compilation of datacollected from lab and/or field operation of exhaust treatment system12. The virtual model may contain data correlating exhaust gas NO:NO₂ratio as reported by sensor 38, bypass valve 20 setting, operatingparameters of power source 10 and/or DOC 26, and an expected exhaust gasNO:NO₂ ratio downstream of DOC 26. A virtual model may enable controller36 to determine, based on sensed exhaust gas NO:NO₂ ratio, power source10 operating parameters, and/or DOC 26 operating parameters, a settingof valve element 22 that will produce a desired exhaust gas NO:NO₂ ratiodownstream of DOC 26. Controller 36 may use a virtual model in an openloop mode of operation of exhaust treatment system 12, as describedbelow.

Sensor 38 may be associated with main passageway 14. Sensor 38 is shown,for example, downstream of DOC 26. One skilled in the art willrecognize, however, that sensor 38 may alternatively or additionallyinclude sensing elements associated with, for example, power source 10,DOC 26, and SCR device 32. Sensor 38 may directly sense a concentrationof NO and NO₂ in an exhaust gas flow, and generate a signal in responsethereto. Alternatively, sensor 38 may sense a NO:NO₂ ratio of an exhaustgas flow and generate a ratio signal in response thereto. Sensor 38 maybe any type of sensor commonly known in the art for sensing NO and NO₂composition.

It is contemplated that sensor 38 may alternatively embody both physicalsensors and a virtual sensor, included in controller 36, that generatesa signal based on a map-driven estimate. Such a virtual sensor mayinclude one or more physical sensing elements associated with, forexample, power source 10 and/or DOC 26. Physical sensing elements maydetect and communicate to controller 36 parameters including, forexample power source fuel/air settings, power source operating speed,power source load, power source fuel injection profile, other powersource operating parameters, and/or DOC 26 operating temperature.Virtual sensor 38 may evaluate the signals received from variousphysical sensors, and, using relationships contained within one or moremaps stored in a memory of controller 36, estimate the expected exhaustgas NO:NO₂ ratio based on the sensed parameters.

Controller 36 may monitor and regulate valve element 22 of bypass valve20 to control the amount of exhaust gas delivered to DOC 26. Controller36 may monitor an actual exhaust gas NOx composition via sensor 38 todetermine a NO:NO₂ ratio, and then adjust valve element 22 to deliver anamount of exhaust gas to DOC 26 necessary to provide SCR device 32 withexhaust gas having a desired ratio of NO:NO₂. In one embodiment, thedesired ratio may be 1:1. Alternatively, if using a virtual sensor 38,controller 36 may monitor operating parameters of power source 10 and/orDOC 26, use the maps stored in memory to estimate an exhaust gas NO:NO₂ratio based on the sensed operating parameters, and then adjust valveelement 22 to deliver an amount of exhaust gas to DOC 26 necessary toprovide SCR device 32 with exhaust gas having a desired NO:NO₂ ratio. Inone embodiment, the desired ratio may be 1:1.

Controller 36 may control a NO:NO₂ ratio using either a closed or openloop scheme. In closed loop operation, controller 36 may measure aNO:NO₂ ratio using sensor 38, determine that the ratio is too low,adjust valve element 22 toward its open position to direct more exhaustgas flow to bypass passageway 24, and then measure a NO:NO₂ ratio again.If controller 36 determines the ratio is still too low, controller 36may open bypass valve 20 further and then measure a NO:NO₂ ratio again,continuing until the desired NO:NO₂ ratio is obtained. Alternatively,controller 36 may measure a NO:NO₂ ratio using sensor 38, determine thatthe ratio is too high, adjust valve element 22 toward its closedposition to direct more exhaust gas flow to DOC 26, and then measure aNO:NO₂ ratio again. If controller 36 determines the ratio is still toohigh, controller 36 may close bypass valve 20 further and then measure aNO:NO₂ ratio again, continuing until the desired NO:NO₂ ratio isobtained.

In open loop operation, controller 36 may measure a NO:NO₂ ratio usingsensor 38, compare that ratio to a desired NO:NO₂ ratio, and then, basedon a virtual model of exhaust treatment system 12 stored in memory ofcontroller 36, adjust valve element 22 to a specific settingcorresponding to the desired NO:NO₂ ratio. For example, when sensor 38reports a NO:NO₂ ratio of 2:1, then controller 36 may use a virtualmodel of exhaust treatment system 12 to determine that valve element 22should be moved to, for example, 25% bypass, so as to obtain a desiredexhaust gas NO:NO₂ ratio for SCR device 32.

FIG. 2 shows a flowchart illustrating an exemplary method of operatingcontrol system 34. FIG. 2 will be described in detail below.

INDUSTRIAL APPLICABILITY

The exhaust treatment system of the present disclosure may be applicableto any power source, including, for example, an engine or a furnace thatbenefits from reduced NOx emissions. In particular, the disclosed systemmay improve reduction of NOx by providing an approximately 1:1 mix of NOand NO₂ to an associated SCR device. The operation of exhaust treatmentsystem 12 will now be explained.

Referring to FIG. 1, air and fuel may be drawn into power source 10 forsubsequent combustion. Fuel may be injected into power source 10, mixedwith the air therein, and combusted by power source 10 to produce amechanical work output and an exhaust gas flow. The exhaust gas flow maycontain a complex mixture of air pollutants composed of gaseousmaterial, which can include oxides of nitrogen (NOx). As this NOx ladenexhaust gas flow is directed from power source 10 through exhausttreatment system 12, DOC 26 may modify a NOx composition of exhaust gasby converting NO to NO₂, and SCR device 32 may remove NO₂ from theexhaust gas flow by conversion to N₂ (Step 100).

During operation of power source 10, controller 36 may determine a ratioof NO:NO₂ based on a measured NO and NO₂ concentration (i.e. based on asignal from sensor 38) (Step 102). Alternatively, controller 36 maydetermine an exhaust gas NO:NO₂ ratio by sensing operating parameters ofpower source 10 and/or DOC 26, and then compare the parameters withrelationships stored in one or maps in controller 36 memory. Forexample, controller 36 may use a map as a lookup table to determine theratio of NO:NO₂ based on sensed power source fuel/air settings, powersource operating speed, power source load, power source fuel injectionprofile, other power source operating parameters, and/or DOC 26operating temperature.

Controller 36 may evaluate the ratio of NO:NO₂ to determine a furthercourse of action by comparing the sensed or determined NO:NO₂ ratio withan expected or desired NO:NO₂ ratio (Steps 104 a and 104 b). Forexample, a desired NO:NO₂ ratio may be 1:1. When controller 36determines the ratio of NO:NO₂ equals 1:1, then controller 36 continuesto determine a ratio of NO:NO₂ based on a measured NO and NO₂concentration (Step 104 a). When controller 36 determines that the ratioof NO:NO₂ is greater than 1:1 (Step 104 b), then controller 36 mayadjust valve element 22 toward its closed position to increase theamount of exhaust gas flowing through DOC 26. Controller 36 may decreasethe amount of exhaust gas flowing through bypass passageway 24 until theratio of NO:NO₂ in the exhaust gas reaches about 1:1 (Step 106).

However, when controller 36 determines that the exhaust gas ratio ofNO:NO₂ is less than 1:1 (Step 104 b), controller 36 may adjust valveelement 22 toward its open position to decrease the amount of exhaustgas flowing through DOC 26. Controller 36 may increase the amount ofexhaust gas flowing through bypass passageway 24 until the ratio ofNO:NO₂ in the exhaust gas reaches about 1:1 (Step 108).

The disclosure sets forth ways in which exhaust treatment system 12 maycontinuously control the conversion of NO to NO₂ in preparation for theSCR process. This control of an exhaust gas flow NO:NO₂ ratio may allowcontroller 36 to maintain a NO:NO₂ ratio at about 1:1. This optimalratio may allow SCR device 32 to operate at maximum efficiency whenconverting NO₂ to N₂. Estimation of a NO:NO₂ ratio based on power sourceoperating parameters and a map may provide a more accurate estimate of aNO:NO₂ ratio, as well as a more rapid estimation of a NO:NO₂ ratio thatenables real-time control of a NO:NO₂ ratio. A rapid, more accurateestimate of a NO:NO₂ ratio may provide emissions from power source 10that are better able to meet stringent standards.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the system of the presentdisclosure without departing from the scope of the disclosure. Otherembodiments will be apparent to those skilled in the art fromconsideration of the specification and practice of the system disclosedherein. It is intended that the specification and examples be consideredas exemplary only, with a true scope of the disclosure being indicatedby the following claims and their equivalents.

1. An exhaust treatment system, comprising: an SCR device; an oxidationdevice located upstream of the SCR device to convert NO to NO₂; anexhaust passageway extending from an exhaust source to the oxidationdevice; a bypass passageway extending from the exhaust passageway at alocation upstream of the oxidation device to the exhaust passageway at alocation downstream of the oxidation device; a valve element configuredto selectively direct exhaust from the exhaust source through theoxidation device and through the bypass passageway; at least one sensorconfigured to sense operating parameters of the exhaust source; and acontroller in communication with the valve element, the controller beingconfigured to move the valve element in response to an estimated ratioof NO to NO₂ based on sensed operating parameters of the exhaust source.2. The exhaust treatment system of claim 1, wherein the controllerincludes a map stored in a memory thereof relating the exhaust sourceoperating parameters to an amount of NO and an amount of NO₂ produced bythe exhaust source.
 3. The exhaust treatment system of claim 1, whereinthe exhaust source operating parameters include at least one of fuel/airsettings, operating speed, load, and fuel injection profile.
 4. Theexhaust treatment system of claim 1, wherein the valve element includesa three-way valve located at the junction of the bypass passageway andthe exhaust passageway upstream from the oxidation device.
 5. Theexhaust treatment system of claim 1, wherein the valve element includesa two-way valve located within the bypass passageway.
 6. The exhausttreatment system of claim 1, wherein: the controller has stored in amemory thereof a virtual model of the exhaust treatment system; and thevalve element is moved based on the estimated ratio of NO to NO₂ in theexhaust gas and the virtual model.
 7. The exhaust treatment system ofclaim 1, wherein a greater amount of exhaust is directed through the DOCwhen the amount of NO₂ in the exhaust is less than the amount of NO inthe exhaust.
 8. The exhaust treatment system of claim 7, wherein agreater amount of exhaust is directed through the bypass passageway whenthe amount of NO₂ in the exhaust is greater than the amount of NO in theexhaust.
 9. The exhaust treatment system of claim 1, wherein theoxidation device includes a substrate coated with a precious metal. 10.A method of treating exhaust, comprising: generating a flow of exhaust;treating at least a portion of the flow of exhaust by a catalyst;directing the flow of exhaust through an SCR device; estimating a ratioof NO to NO₂ in the flow of exhaust based on sensed operating parametersof a power source that generates the flow of exhaust; and changing anamount of the at least a portion in response to the estimation.
 11. Themethod of claim 10, wherein estimating includes referencing a knownrelationship between power source operating parameters and theproduction of NO and NO₂.
 12. The method of claim 11, wherein the powersource operating parameters include at least one of fuel/air settings,operating speed, load, and fuel injection profile.
 13. The method ofclaim 12, wherein changing an amount of the at least a portion includesestimating the amount of the at least a portion based on the ratio of NOto NO₂ and the predicted behavior of the catalyst.
 14. The method ofclaim 10, further including increasing the amount of the at least aportion when an amount of NO in the exhaust directed to the SCR deviceexceeds an amount of NO₂ in the exhaust directed to the SCR device. 15.The method of claim 14, further including decreasing the amount of theat least a portion when the amount of NO₂ in the exhaust directed to theSCR device exceeds the amount of NO in the exhaust directed to the SCRdevice.
 16. A power system, comprising: a power source configured tocombust a fuel/air mixture and generate power and a flow of exhaust; anSCR device; an exhaust passageway fluidly communicating the power sourcewith the SCR device; an oxidation device to convert NO to NO₂ located inthe exhaust passageway between the power source and the SCR device; abypass passageway extending from the exhaust passageway at a locationupstream of the oxidation device to the exhaust passageway at a locationdownstream of the oxidation device; a valve element configured toselectively direct exhaust from the power source through the oxidationdevice and through the bypass passageway; at least one sensor configuredto sense operating parameters of the exhaust source; and a controller incommunication with the valve element, the controller being configured tomove the valve element in response to an estimated ratio of NO to NO₂based on sensed operating parameters of the power source.
 17. The powersystem of claim 16, wherein the controller includes a map stored in amemory thereof relating power source operating parameters to an amountof NO and an amount of NO₂ produced by the power source.
 18. The powersystem of claim 17, wherein the power source operating parametersinclude at least one of fuel/air settings, operating speed, load, andfuel injection profile.
 19. The power system of claim 16, wherein agreater amount of exhaust is directed through the DOC when an amount ofNO₂ in the exhaust is less than an amount of NO in the exhaust.
 20. Thepower system of claim 19, wherein a greater amount of exhaust isdirected through the bypass passageway when the amount of NO₂ in theexhaust is greater than the amount of NO in the exhaust.